Radio interference is a major cause of performance degradation in wireless radio systems. To mitigate radio interference and aid performance, state-of-the-art radio cellular systems have adopted Inter-Cell Interference Coordination (ICIC) schemes. In the related art LTE system, for instance, two forms of ICIC are supported: frequency domain ICIC (adopted in LTE Rel. 8-9); and time domain ICIC (adopted from LTE Rel. 10).
Frequency domain ICIC relates to the usage of radio resources in the frequency domain and/or power adaptation. Current methods include:                Full frequency reuse, (the basic operating mode of the LTE system) in which each base station uses the entire frequency spectrum with uniform power distributed across the system bandwidth, thereby creating strong interference to cell edge users.        Hard frequency reuse, (used in the related art GSM and LTE Rel. 8-9) in which each base station operates in one out of a set of non-overlapping portions of the available frequency spectrum in such a way that neighbouring base stations do not use the same set of frequencies. While this minimizes the interference at the cell-edge, the overall spectral efficiency is reduced by a factor equal to the reuse factor.        Fractional frequency reuse in which the available frequency spectrum is divided into two portions: a portion common to all base stations used for scheduling cell-centre users, and a second portion that is further divided among base stations in a hard frequency reuse manner and used to schedule transmission to/from cell-edge users.        Soft frequency reuse which enables base stations to transmit in the entire frequency spectrum with different power levels: higher transmission power in the portion of the spectrum where cell-edge users are scheduled; lower transmission power in the portion of spectrum where cell-centre users are scheduled.        
Time domain ICIC consists in periodically muting the transmission of a base station in certain time-frequency resources to enable a further base station to serve mobile stations suffering severe interference in the muted radio resources. The related art LTE system introduced Almost Blank Subframes (ABS), i.e., downlink subframes where only the necessary signals to avoid radio link failure or to maintain backward compatibility are transmitted, including common reference signals (except subframes configured as MBSFN), Primary and Secondary Synchronization Signals (PSS/SSS), Physical Broadcast Channel (PBCH), SIB-1 and paging with their associated PDCCH.
Time domain muting patters are configured semi-statically by means of bitmaps of length 40, i.e., spanning up to four radio frames, signalled between eNodeBs over the X2 interface. Mobile stations in a victim cell are then categorized into two groups:                Mobile stations affected by interference form a cell using ABS, which shall preferably be scheduled in correspondence of a muted subframe from said cell;        And mobile stations that are not affected by the interference produced by a neighbouring cell using ABS, which can be scheduled freely in any subframe.        
The above categorization is done by comparing channel quality (CSI) feedback from mobile stations in muted and non-muted subframes of a neighbouring cell.
Despite the term ICIC suggests a coordination among base stations, de facto there is no coordination in the way ABS patterns are locally decided by a base station or used by a victim base station to schedule mobile stations in its coverage area. Coordination is limited to exchanging information among base stations, such as the ABS pattern of a base station or interference indicators (e.g., the overload indicator OI and the high interference indicator HII in the LTE system) reporting whether specific radio resources suffer high, low or medium interference.
Another method to mitigate interference in the downlink of a cellular radio system is by transmission beamforming. In essence, in a multi-antenna system a base station may fine-tune the transmission of a downlink radio signal into a narrow beam in the direction of the intended receiver by mean of transmission precoding thereby reducing the interference caused to other mobile nodes in the cell coverage area.
ABS adopted in LTE Rel-10 to mitigate interference for cell-edge users, comprise Time-Domain Muting (TDM) patterns of data transmission in downlink subframes. The muting pattern of an aggressor cell (typically a macro base station) is signalled over the X2 interface to a neighbouring victim cell (typically pico base stations within the macro-cell coverage area), so that the latter can schedule mobile nodes suffering strong interference from the aggressor cell in ABS subframes of the aggressor cell. Mobile nodes in the coverage are of the victim cell are configured to perform channel quality (CSI) measurements in correspondence of ABS and non-ABS resources to enable the serving cell determine whether the mobile station is affected by strong interference from the aggressor cell. The TDM muting patterns and the scheduling decisions are independently determined by the aggressor cell and the victim cell respectively.
According to another prior art solution a method is proposed for a mobile station to determine whether it is being interfered by a neighbouring cell. The method comprises the detection that an interference condition occurs when a quality of the neighbouring cell becomes better than a quality of the serving cell by a threshold or for a certain period. The method does not consider how the ABS patterns are created and the impact on such patterns on the user scheduling.
According to yet another prior art solution the concept of ABS is extended to transmission beam coordination. In particular, it provides a method for beam coordination between an aggressor base station and a victim base station, where the interference created by a beam transmitted by an aggressor base station is measured by mobile stations in the served area of a victim base station. Depending on the measured interference, a restriction of use of radio resources is determined for the aggressor base station, which is used by the victim base station to schedule users in restricted radio resources as in the ABS case. A mobile station in the victim cell reports which beams are deemed as interfering and eventually a corresponding channel quality measure. Based on this information, the victim cell performs a ranking of interfering beams and determines a scheduling of the served users based on muted/restricted resources signalled by an aggressor cell. The aggressor cell determines a restriction of usage of radio resources based on the ranking of interfering beams at the victim cell. Coordination is limited to the exchange of information among a victim and an aggressor base stations (namely, a ranking of interfering beams and restriction of resource use, respectively), whereas local decisions are taken at the victim and the aggressor base stations (related to ranking of interfering beams and scheduling of served user terminals and the restriction of usage of radio resources for ranked interfering beams, respectively) without coordination.